huh. i didnt have this problem at all. im very sorry he is having such a hard time!
first of all, make sure that there hasnt been any clothing issues. are there zippers poking into his skin ? tags? seams? dirt? check the car seat for pokey parts. make sure above all that the clothes you put him in are simple, to avoid getting some parts poking into his skin or something.
also the diaper isnt too tight... etc.
then make sure the straps are at the right spots on baby. maybe move the bottom buckle forward in case its squishing precious bottom parts. NEVER loosen the straps to "make it more comfortable" as that is DANGEROUS (but as a day care provider i see it OVER AND OVER)...
if you have a white noise cd to play in the car or something try that. my son always calmed down to white noise (specifically the vacuum which we wore OUT until i found "For Crying Out Loud" a white noise cd mix)..
what else.. is it possible for someone to sit in the backseat with him?? is there sun in his eyes? is there a reflection of sunshine? is he too hot? too cold?
wow. this is a tough one. i really hope that he gets better. dont try to fill his tummy too much as i can imagine that the straps can make a full tummy pretty uncomfortable.. and the burping thing would make it hurt too...
something i remember from in the hospitol.. our nurse took the car seat at the head and the foot, held the side to her waist or upper thighs, and swung it right and left, as you turn your body with it right and left, you know what i mean??
that worked like a charm from the very first time we ever had him in a car seat. and now that i think about it, we had to do it almost every time he was in it too... so maybe try that.
let me know if it works.